1. Field of the Invention
Exemplary embodiments of the present invention generally relate to an endless belt member, a transfer unit incorporating the endless belt member, and an image forming apparatus incorporating the endless belt member.
2. Discussion of the Related Art
Full-color image forming apparatuses for electrophotographic printing generally perform either a direct transfer operation or an indirect transfer operation. In the indirect transfer operation, which is a two-step operation, a toner image formed on an image carrier that contacts an intermediate transfer belt is transferred onto an outer circumferential surface of the intermediate transfer belt by an electric field supplied by a transfer bias unit, in an operation that is referred to as primary transfer. Then, the toner image retained by the intermediate transfer belt is transferred onto a transfer member or a recording medium conveyed along the outer circumferential surface of the intermediate transfer belt, in an operation referred to as secondary transfer. Through the primary and secondary transfers, ultimately a full-color toner image is formed on a recording medium.
For example, one related-art image forming apparatus includes a multi-layer intermediate transfer belt composed of a high-resistivity surface layer that forms an outer circumferential surface for carrying a toner image thereon and a medium-resistivity base layer that forms an inner circumferential surface of the multi-layer intermediate transfer belt to which a transfer bias that has an opposite polarity to a toner charge polarity is applied. Compared with a medium-resistivity surface layer, such a high-resistivity surface layer can provide better charge retention of the transfer bias having the opposite polarity after transfer.
If the surface layer retains only a small residual charge of the transfer bias having the opposite polarity after transfer, a toner image formed on the outer circumferential surface of the multi-layer intermediate transfer belt cannot be retained thereon electrostatically, that is, some of toner particles are scattered over the outer circumferential surface of the multi-layer intermediate transfer belt, adversely affecting the quality of the toner image. By contrast, a large residual charge of the transfer bias having the opposite polarity on the surface layer after transfer can hold the toner image formed on the outer circumferential surface of the multi-layer intermediate transfer belt with electrostatic force, thereby preventing the above-described toner scattering.
Moreover, at transfer, under a related-art constant current control, resistivity of the intermediate transfer belt is affected by such environmental factors as ambient temperature and relative humidity in the image forming apparatus. Changes in resistivity can change the size of the transfer bias having a polarity opposite the charge polarity of toner that may be applied at transfer, causing the charge potential of the intermediate transfer belt to vary as a result.
As compared with a reference resistivity under given reference temperature and humidity conditions, the resistivity of the intermediate transfer belt may decrease with high temperature and high relative humidity, which reduces the size of the transfer bias to be applied for transfer under constant-current control and consequently decreases the charge potential of the intermediate transfer belt. Further, a reduction in resistivity of the intermediate transfer belt can decrease the size of the opposite electric charge remaining on the surface layer of the intermediate transfer belt compared to the residual charge at the given reference temperature and relative humidity. Therefore, in addition to the reduced charge potential of the intermediate transfer belt, the size of the electrical charge with the opposite polarity remaining on the surface layer of the intermediate transfer belt after transfer also decreases. Due to these decreases in the sizes of the electrical charge and charge potential, the toner image formed on the outer circumferential surface of the intermediate transfer belt cannot be held with the required electrostatic force, and therefore toner scattering can easily occur.
By contrast, the resistivity of the intermediate transfer belt may increase under conditions of lower temperature and lower relative humidity, which increases the size of the transfer bias to be applied for transfer under constant-current control, and consequently increases the charge potential of the intermediate transfer belt. Further, an increase in resistivity of the intermediate transfer belt can increase the charge of the opposite polarity remaining on the surface layer of the intermediate transfer belt compared to that under the given reference temperature and relative humidity. Therefore, in addition to the increased charge potential of the intermediate transfer belt, the size of the electrical charge with the opposite polarity remaining on the surface layer of the intermediate transfer belt after transfer also increases. Due to these increases in the amounts of the electrical charge and charge potential, the toner image formed on the outer circumferential surface of the intermediate transfer belt can be held with the required electrostatic force, and therefore toner scattering can be prevented. However, it is known that, as the size of electric charge remaining on the surface layer of the intermediate transfer belt after transfer increases, residual images can appear more easily.
As described above, even when a multi-layer intermediate transfer belt having a high-resistivity surface layer and a medium-resistivity base layer is used, the occurrence of toner scattering cannot be completely eliminated. Further, residual images can be generated.